Renamed Ptr types

Renamed Ptr types
        FooPtr / constFooPtr    (deprecated old)
to
        Foo_Ptr / Foo_constPtr  (new)

To be more consistent with Ptr types declared within a class as
        Foo::Ptr / Foo::constPtr

zypp/base/PtrTypes.h:
Renamed ImplPtr (we have too much Impl's) into 'RW_pointer'.
This indicates it's smart pointerness. Updated doc for it and
added code example.